As is known, the NAVSTAR Global Positioning System is a continuous, space-based navigation system that provides any suitably equipped user with highly accurate three-dimensional position, velocity and time information anywhere on or near the earth. GPS is essentially a satellite ranging system, wherein any user, such as an aircraft, can estimate its position by measuring its range to at least four GPS satellites and, using triangulation techniques, estimate its position and local clock error. The GPS system operates by timing how long it takes a radio frequency signal emitted by the satellites to reach the user and then calculating the distance based on that time measurement. While the GPS system can generally be used to provide highly accurate position information, the ultimate accuracy is degraded by the sum of several sources of error, including delays in the transmission of the signal as it travels through the ionosphere and troposphere, ephemeris errors (i.e., variations in the altitude, position and velocity of the GPS satellites), and variations between the receiver clocks and the satellite clocks. As is known, the accuracy of a GPS positioning system can be enhanced by using a technique called differential GPS, which can assist in determining the inaccuracies in the signal transmitted by the GPS satellites and provide other GPS receivers in the local area with a set of differential corrections that the other GPS receivers can use to correct their position solutions. The basic differential GPS concept is generally valid because the range from the GPS satellites to the receivers is sufficiently large that any errors measured by one receiver will be almost exactly the same for any other receiver in the same locale. Generally, a single differential correction factor will account for most errors in the GPS system, including receiver and/or satellite clock errors, variations in the positions of the satellite(s), and ionospheric and atmospheric delays.
A differential GPS landing system typically includes one or more GPS receivers located adjacent to one another at fixed, known positions, which are capable of receiving signals broadcast from a number of GPS satellites. The signals broadcast from each GPS satellite include emphemeris and course almanac data, which may be used to determine the location of the satellite. The signals also include a pseudorandom code, which can be used to determine the transmission time that the particular GPS satellite broadcast its data. This transmission time may then be used to calculate a pseudorange (i.e., a range that has not been corrected for errors in synchronization between the satellite's clock and the receiver's clock) between the GPS satellite and the GPS receiver. The GPS receiver calculates a pseudorange to each satellite in view by monitoring a pseudorandom code transmitted by the satellite and comparing this code to its own reference code to determine the transmission duration of the signal broadcast from each GPS satellite in view and received by the GPS receiver. A calculated true range between the receiver and each satellite may be determined by using the emphermis and/or course almanac data, together with stored information on the orbits and velocity of the satellites, and the known location of the receiver. The difference between the highly accurate calculated range and the computed pseudorange represents a differential range error for that given satellite's broadcast data. This difference may be broadcast to each GPS receiver in the locale to process and apply to its computed pseudoranges in order to improve the accuracy of the pseudorange computed by that GPS receiver.
Applying these concepts to a GPS system that assist aircraft in navigating and landing, a set of differential corrections may be determined by a differential GPS landing system and broadcast for reception by all aircraft within broadcast range (e.g., around a particular airport). The aircraft, using their own GPS receiver, may then receive the information broadcast from the GPS satellites, calculate a set of pseudorange range values from the information received from the GPS satellites, and receive, process and apply the differential corrections broadcast from the GPS landing system. It thereby improves the accuracy and integrity of the aircraft position determination. If the broadcast differential corrections are sufficiently accurate and reliable, a differential GPS landing system may be used in connection with a Special Category I (SCAT-I) approach and landing operation.
One of the more difficult problems in a local area differential GPS landing system is integrity monitoring of the broadcast corrections. It will be appreciated that the differential corrections that are broadcast by the differential GPS landing system must be extremely accurate and reliable and that the landing system include means for preventing the transmission of erroneous, unsafe correction data. Thus, based on RTCA DO-217 requirements for SCAT-I approach and landing systems (as expressed in the Minimum Aviation System Performance Standards DGNSS Instrument Approach System: Special Category I (SCAT-I), RTCA/DO-217, prepared by SC-159, RTCA Incorporated, Aug. 27, 1993 (including Changes 1 and 2)), a 1.times.10.sup.-7 error bound must be placed on the differential corrections with an integrity of this bound equal to 1.times.10.sup.-8.
One approach for enhancing the integrity of the calculated differential corrections is to provide redundant processors that also receive satellite data from a number of GPS receivers. Each processor calculates its own set of differential corrections using data received from the plurality of GPS receivers, that are then averaged together in some fashion to form the overall system differential corrections. For example, each processor may initially form independent sets of differential corrections for the data received by each receiver, combine these corrections into an overall processor set of corrections, which may then be combined with the corrections calculated by other processors. A drawback to this approach is that each processor is essentially performing the identical calculations on substantially identical data and, thus, cannot account for errors which may occur because of common mode failures. Prior approaches typically assume that each processor will receive and process all data received by the receivers included in the differential GPS landing system. This approach may be further enhanced by providing an integrity monitoring function that ensures that the overall system differential corrections fall with a predefined threshold. However the predefined threshold must be derived from knowledge of the statistics of the error at a particular time with a particular hardware configuration. The statistics of the error can change over time, thus invalidating the original predefined threshold.
Thus, there is a continuing need for a differential GPS landing system that produces differential corrections that improve ranging accuracy in order to support precision approaches, while also providing timely integrity information of the data broadcast to aircraft for validation and processing.